Bizarre planets with internal layers of diamond many kilometres thick may form in carbon-rich areas of the galaxy, a new study suggests.
The diamond-rich planets could form from the dusty protoplanetary discs found around many stars, if they are rich in carbon and poor in oxygen
, says Marc Kuchner at Princeton University, New Jersey, US.
Composed largely of heat-resistant carbides and graphite - as well as diamond - these planets could withstand much higher temperatures than terrestrial planets or gas giants, he says. This might account for giant non-gas planets found surprisingly close to other stars.
Although terrestrial life is based on carbon, the Earth is made largely of silicates
. It contains only about 44 parts per million of carbon. However, the element is 1000 times more common in chondrite meteorites
, which originated in the Solar System, though probably in a different region of the protoplanetary disc. Curious about these compositional differences, Kuchner and Sara Seager of the Carnegie Institution of Washington, US, studied prospects for building planets from the carbon-rich material of the meteorites.
The key factor behind the difference between carbon and silicate-based planets is the chemistry of the protoplanetary disc. Changing the ratio of carbon to oxygen "makes a huge difference to what condenses out"
, says Kuchner.
levels produce silicate-based planets like the Earth, Venus
, and Mars
. But high carbon levels cause carbon compounds to condense out of the disc and clump together, producing carbon-based planets
Like the Earth, a carbon-based planet would have a metallic iron core, but the surrounding layers would be different, they suggest. Above the core would be a thick layer of silicon and titanium carbides, extremely hard ceramics known for their impressive heat-resistance. Graphite would form above the carbide, with high pressure converting the bottom of the graphite layer to a shell of crystalline diamond.
A carbon-based planet would be oxygen poor, with a tarry hydrocarbon surface and an atmosphere rich in hydrocarbons and carbon monoxide
. Future planet-hunting telescopes
might be able to spot one by looking for carbon monoxide's distinctive spectral signature.
As long as water could somehow be delivered to their surfaces after they formed, such planets should be able to support life forms, Kuchner told New Scientist
"Life on a carbon planet would be strange," he says. Oxygen-containing materials would be flammable in its hydrocarbon atmosphere, so metabolism might be the inverse of terrestrial life - burning oxygen as food rather than carbon compounds.
Older stars tend to spew more carbon, so as the Universe
ages, carbon planets may become more likely, he says. They may already be more common near the ancient galactic nucleus, says Kuchner.